WHO says bacon causes cancer?

by Neha PancholiNote: Here at the PSPG blog, we like to feature writing from anyone in the Penn community interested in the science policy process or science for general interest. This is the 1st in a series of posts from new authors. Interested is writing for the blog? Contact us!

The daily meat
consumption in the United States exceeds that of almost every other country1.
While the majority of meat consumed in the United States is red meat2,
the consumption of certain red meats has decreased over the past few decades
due to associated health concerns, such as heart disease and diabetes1,2.
In October, the World Health Organization (WHO) highlighted another potential
health concern for red meat: cancer.

The announcement concerned both red and processed meat. Red
meat is defined as unprocessed muscle meat from mammals, such as beef and pork3.
Processed meat– generally red meat –has been altered to improve flavor through
processes such as curing or smoking3. Examples of processed meat
include bacon and sausage. The WHO confirmed that processed meat causes cancer
and that red meat probably causes cancer. Given the prevalence of meat in the American diet, it was not surprising
that the announcement dominated headlines and social media. So how exactly did
the WHO decide that processed meat causes cancer?

The announcement by the WHO followed a report from the
International Agency for Research on Cancer (IARC), which is responsible for
identifying and assessing suspected causes of cancer. The IARC evaluates the typical level of exposure to a suspected agent,
results from existing studies, and the mechanism by which the agent could cause
cancer.

After a review of
existing literature, the IARC classifies the strength of scientific evidence linking the suspected
cancer-causing agent to cancer. Importantly, the IARC determines only whether
there is sufficient evidence that something cancause cancer. The IARC does
not evaluaterisk, meaning that it
does not evaluate how carcinogenic something is. The IARC classifies the
suspected carcinogen into one of the following categories4:

Group 1 – There is convincing evidence linking the agent to cancer in humans.
The agent is deemed carcinogenic.

Group 2A – There is sufficient evidence of cancer in animal models, and there is
a positive association observed in humans. However, the evidence in humans does
not exclude the possibility of bias, chance, or confounding variables. The
agent is deemed as a probable carcinogen.

Group 2B – There is a positive association in humans, but the possibility of
bias, chance, or confounding variables cannot be excluded. There is inadequate
evidence in animal models.

This category is also used when there is
sufficient evidence of cancer in animal models, but there is not an association
observed in humans. The agent is a possible carcinogen.

Group 3 – There is inadequate evidence in humans and animals. The agent cannot be classified as carcinogenic or
not carcinogenic.

Group 4 – There is sufficient evidence to conclude that the agent is not carcinogenic in humans or in
animals.

The IARC reviewed
over 800 studies that examined the correlation between consumption of processed
or red meat and cancer occurrence in humans. These types of studies, which
examine patterns of disease in different populations, are called
epidemiological studies. The studies included observations from all over the
world and included diverse ethnicities and diets. The greatest weight was given
to studies that followed the same group of people over time and had an
appropriate control group. Most of the available data examined the association
between meat consumption and colorectal cancer, but some studies also assessed
the effect on stomach, pancreatic, and prostate cancer. The majority of studies
showed a higher occurrence of colorectal cancer in people whose diets included
high consumption of red or processed meat compared to those who have low
consumption. By comparing results from several studies, the IARC determined
that for every 100 grams of red meat consumed per day, there is a 17% increase
in cancer occurrence. For every 50 grams of processed meat eaten per day, there
is an 18% increase.The average red meat consumption for those who
eat it is 50-100 grams per day.3

The IARC also
reviewed studies that examined how meat could cause cancer. They found strong
evidence that consumption of red or processed meat leads to the formation of
known carcinogens called N-nitroso
compounds in the colon. It is also known that cooked meat contains two types of
compounds that are known to damage DNA, which can lead to cancer. However,
there is not a direct link between eating meat containing these compounds and
DNA damage in the body.3

Based on the strong
evidence demonstrating a positive association with consumption of processed
meat and colorectal cancer, the IARC classified processed meat as a Group 1
agent3. This means that there is sufficient evidence that
consumption of processed meat causes cancer.

There was a
positive association between consumption of red meat and colorectal cancer in
several epidemiological studies. However, the possibility of chance or bias
could not be excluded from these studies. Furthermore, the best-designed
epidemiological studies did not show any association between red meat
consumption and cancer. Despite the limited epidemiological evidence, there was
strong mechanistic evidence demonstrating that red meat consumption results in
the production of known carcinogens in the colon. Therefore, red meat was
classified as a probable carcinogen (Group 2A)3.

It will be
interesting to see how the WHO announcement affects red meat consumption in the
United States and worldwide. But before swearing off processed and red meat
forever, there are a few things to consider.

First, it is
important to bear in mind that agents classified within the same group have
varying carcinogenic potential. Processed meat was classified as a Group 1
agent, which is the same classification for tobacco smoke. However, estimates
by the Global Burden of Disease Project attribute approximately 34,000 cancer
deaths per year to consumption of processed meat5. In contrast, one
million cancer deaths per year are due to tobacco smoke5. While the
evidence linking processed meat to cancer is strong, the risk of cancer due to
processed meat consumption appears to be much lower than other known
carcinogens. Second, the IARC did not evaluate studies that compared vegetarian
or poultry diets to red meat consumption5. Therefore, it is unknown
whether vegetarian or poultry diets are associated with fewer cases of cancer. Finally,
red meat is high in protein, iron, zinc, and vitamin B123. Thus,
while high red meat consumption is associated with some diseases, there are also
several health benefits of consuming red meat in moderation. Ultimately, it
will be important to balance the risks and benefits of processed and red meat
consumption.

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